Diesel fuel is a refined petroleum product which is burned in the engines powering most of the world's trains, ships, and large trucks. Petroleum is, of course, a non-renewable resource of finite supply. Acute shortages and dramatic price increases in petroleum and the refined products derived from petroleum have been suffered by industrialized countries during the past quarter-century. Further, diesel engines emit relatively high levels of certain pollutants, especially particulates. Accordingly, extensive research effort is now being directed toward replacing some or all petroleum-based diesel fuel with a cleaner-burning fuel derived from a renewable source such as farm crops.
Vegetable oils have been directly added to diesel fuel in an attempt to replace at least a portion of the diesel fuel. These vegetable oils are composed mainly of triglycerides, and often contain small amounts (typically between 1 and 10% by weight) of free fatty acids. Some vegetable oils also contain small amounts (typically less than a few percent by weight) of mono- and di-glycerides.
Triglycerides are esters of glycerol, CH.sub.2 (OH)CH(OH)CH.sub.2 (OH), and three fatty acids. Fatty acids are, in turn, aliphatic compounds containing 4 to 24 carbon atoms and having a terminal carboxyl group. Diglycerides are esters of glycerol and two fatty acids, and monoglycerides are esters of glycerol and one fatty acid. Naturally occurring fatty acids, with only minor exceptions, have an even number of carbon atoms and, if any unsaturation is present, the first double bond is generally located between the ninth and tenth carbon atoms. The characteristics of the triglyceride are influenced by the nature of their fatty acid residues. Some of the more common fatty acid residues found in naturally occurring fats and oils are listed in Table 1:
TABLE 1 ______________________________________ Common Fatty Acid Radicals Common Name Chemical Name ______________________________________ Caprylic Octanoic Capric Decanoic Lauric Dodecanoic Myristic Tetradecanoic Palmitic Hexadecanoic Stearic Octadecanoic Caproleic 9-Decanoic Lauroleic 9-Dodecanoic Oleic 9-Octadecanoic Linoleic 9,12-Octadecadienoic Linolenic 9,12,15-Octadecatrienoic ______________________________________
When pure vegetable oils are used as a fuel source in diesel engines, they often cause excessive engine wear and fuel injector coking, and have high smoke values. Further, their viscosity is much higher than petroleum based diesel fuel.
Various attempts have been made to prepare microemulsions of oils and alcohols using a surfactant, and use these microemulsions as alternative fuels. A limitation of these emulsions is that they may not be stable in extreme temperatures, and the alcohol may tend to absorb water from the air, requiring the use of special storage tanks to keep the fuel dry.
In an effort to overcome some of the problems associated with using pure vegetable oils, several attempts have been made to use fatty acid methyl esters. These esters are typically prepared by completely transesterifying triglycerides, the major component in fats and oils, with methanol, in the presence of an acid or base catalyst.
In Europe, large quantities of fatty acid methyl esters are produced by reacting rapeseed oil with methanol in the presence of a strong base.
The glycerol by-product of this reaction is immersible with the methyl esters and is separated. The fatty acid methyl ester-rich stream is washed with water and then blended with diesel fuel at varying ratios. Unfortunately, the reaction is relatively hazardous and produces large amounts of waste products. Furthermore, fuels based on fatty acid methyl esters are not without their adverse effects. Some research indicates that they cause higher emissions of nitrogen oxides (NO.sub.x), increased wear on engine components, and fuel injector coking. An article entitled "Progress in Diesel Fuel from Crop Oils," AgBiotechnology, (1988), indicated that pure vegetable oils and methyl esters both tend to choke the engine with carbon deposits. The authors indicated that diesel engines could probably be run for a single season with these vegetable oils or fatty acid methyl esters, but the engines would then have to be torn down and cleaned up to remove the deposits.
When fatty acid methyl esters are burned, they do not provide as much power as when petroleum diesel is burned (See, for example, Jori, et al., "Comparative test with different biodiesel fuels in tractor engine," Hungarian Agricultural Engineering, 6:7, 27-28 (1993). Often, diesel engines need to be retuned in order to run efficiently on biodiesel.
Fatty acid ethyl esters have also been used as a biodiesel fuel. However, even fatty acid ethyl esters produce less power when burned than petroleum-based diesel fuel. Duedraogo, et al., Diesel engine performance tests using oil from Jatrophacurcas L,"Agricultural Mechanization in Asia, Africa and Latin America, 22(4):25-29, 32 (1991), observed a 19 percent loss of power when using a fatty acid ethyl ester, as compared with conventional diesel fuel. Ethyl esters have many of the same problems associated with the methyl esters when used as fuel or fuel additives.
There remains a need for an improved fuel derived from renewable sources that can be blended with diesel fuel, and that does not damage an engine on long term use.
It is therefore an object of the present invention to provide an alternative fuel source that does not contain significant amounts of fatty acid methyl esters.
It is a further object of the present invention to provide an alternative fuel source that can run on existing engines without needing to retune the engines.
It is still a further object of the present invention to provide a fuel additive that, when combined with diesel fuel, does not damage the engine on long term use.